Not applicable.
(1) Field of the Invention
The present invention generally relates to materials and methods for ultra-high density recording. More particularly, this invention relates to ferromagnetic films having thicknesses of a few monolayers and perpendicular magnetic anisotropy, and to a process for increasing bit density in a magneto-optical recording medium formed of such a film. The process entails introducing linear strain defects into the film to smooth and stabilize the walls of the magnetic domains of the film, thereby arresting the domain wall motion (reduced velocity) under application of a magnetic field, reducing temporal magnetic noise due to motion of magnetic domains, and reducing spatial magnetic noise due to domain wall roughness.
(2) Description of the Related Art
Stable magnetic structures on nanometer-length scales are paramount for ever denser and robust storage of information as the superpararnagnetic limit is approached, at which magnetic energy contained in a tiny bit can no longer compete with the thermal energy and spontaneous magnetization reversal processes will wipe out stored data. It is believed that the onset of thermal effects may shift to much higher bit densities using perpendicular magnetic polarization of nanoscale ferromagnetic films only a few atomic layers (monolayers) thick to maintain large magnetic anisotropy normal to the film plane. In magneto-optic media formed of such films, bit-writing can be performed with local probes that may be thermally assisted by current or a laser beam that raises the local temperature to the vicinity of the Curie temperature (Tc), resulting in the formation of a reversed magnetic domain with a rough wall. Such films make magneto-optical memories appear particularly promising for ultrahigh-density recording on portable disks, with bit densities of about 100 Gbit/inch2 having been demonstrated using recent advances in bit writing and reading techniques. However, the ultimate density of such xe2x80x9cupxe2x80x9d-(xe2x80x9cdownxe2x80x9d-) magnetic domains (bits) will be set by the domain wall roughness and temporal stability (mobility), which are controlled by the inherent disorder in ultra-thin monolayer films.
Magnetization reversal processes in finite-size magnetic materials occur by nucleation of reversed magnetic domains and by their expansion via displacements of domain walls. In thin films, these processes are largely controlled by defects which can act as nucleation and pinning sites. As a result, domain walls nucleate as rare events on random sites. FIG. 1 shows two neighboring domains of an ultra-thin Pt/Co/Pt trilayer film using magneto-optical polar Kerr imaging. As known in the art, a large uniaxial perpendicular anisotropy can be sustained in the Pt/Co/Pt film as a result of a large interface contribution arising from the orbital overlap of Co-3d and Pt-5d wavefunctions. The trilayer stack was electron-beam evaporated at about 190xc2x0 C. on a 200 xc3x85 (111) Pt buffer layer deposited on glass or SiNx/Si substrates. The walls are very rugged in appearance, with noticeable xe2x80x9coverhangsxe2x80x9dxe2x80x94regions where domain wall excursions from the average wall position are multivalued. Overhangs appear when domain walls surround unfavorable regions rather than xe2x80x9cinvadexe2x80x9d or slither over them. Magnetization reversal proceeds by domain growth and as magnetic field (H) increases beyond the coercivity (Hc) of the film, here, about 750 Oe for the Pt/Co/Pt film. During domain growth, the outward motion of the domain walls becomes increasingly swift. The disorder landscape xe2x80x9cfingerprintsxe2x80x9d both the dynamics and the structure of the walls. In a quenched (time-independent) random pinning potential (e.g., arising from atomic scale imperfections at the film-substrate interface), the walls are expected (in a random field Ising spin system) to be self-affine and have been characterized by a fractal dimension. The extended nucleation sites can provide an initial domain shape anisotropy (elongated domains), but being short-range they do not reduce domain wall velocity or affect roughness on larger scales which evolves with time.
From the above, it can be seen that the ultimate bit density that can be achieved with ultra-thin ferromagnetic films having perpendicular magnetic anisotropy is limited by the magnetization reversal process, which results in magnetic domain walls that are very rugged and grow rapidly. Domain wall roughness also leads to spatial magnetic noise, while domain wall instability causes temporal (time-domain) magnetic noise due to motion of magnetic domains. Accordingly, it would be desirable to provide a method by which the domain walls of ultra-thin ferromagnetic films with perpendicular magnetic anisotropy can be made smooth and their velocity reduced during magnetization reversal.
The present invention provides a ferromagnetic film suitable for ultra-high density perpendicular recording, and a process for producing the film that achieves greater bit density and lower temporal and spatial noise. The process generally entails the use of a film of ferromagnetic material characterized by perpendicular magnetic anisotropy and a plurality of magnetic domains defined by domain walls perpendicular to a major surface of the film. According to the invention, the ferromagnetic film is formed on a surface of a substrate to have a linear strain defect, which has been determined to smooth and stabilize the domain walls during subsequent magnetization reversal of the ferromagnetic material. The smoother domain walls achieved with the invention promotes bit density and controls spatial magnetic noise, while greater domain wall stability serves to arrests domain wall motion (reducing velocity) and control temporal magnetic noise due to motion of magnetic domains.
The invention encompasses any suitable method by which a linear strain defect can be incorporated into a ferromagnetic film to produce a strain field having a long range and unique direction. Particular methods identified include the use of patterned grooves formed on the substrate prior to depositing the film, and the application of a uniaxial force when depositing the film and subsequent release of the force. Suitable films for use with the invention are typically a few monolayers thick, and may contain multiple layers of different materials.
In view of the above, the present invention can be seen to make possible stable nanometer-length magnetic structures for denser and more robust storage of information. Such magnetic structures are robust as a result of using perpendicular magnetic polarization to withstand thermal energy and spontaneous magnetization reversal processes, while the density and stability of such magnetic structures are promoted by controlling wall roughness and temporal stability of their magnetic domains. As a result, the present invention makes possible a recording medium having a greater bit density than writable magneto-optical recording mediaxe2x80x94possibly in excess of 1 Tbit/inch2xe2x80x94and is suitable for use in a wide variety of applications, including compact discs for personal computers.
Other objects and advantages of this invention will be better appreciated from the following detailed description.